In-Situ Studies of Silicon Oxidation
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IN-SITU STUDIES OF SILICON OXIDATION J.M. GIBSON*, AND M. Y. LANZEROTTI** *AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 **Dunster House, Harvard University, Cambridge, MA 02138 ABSTRACT Using a UHV transmission electron microscope we have examined the initial stages of Si oxidation. Using the surface-sensitive forbidden 1/3 reflection on Si (111) we have imaged surface steps at various stages of oxidation, including buried Si/Si0 2 interface formation. INTRODUCTION The concept of forbidden surface sensitive Bragg reflections was developed theoretically by Lyncht 1 ] and experimentally by Chernst 2] over a decade ago. Nevertheless, the promise of using these peculiar diffraction features to image surfaces in a wide variety of materials has proven difficult to realize. In this paper we discuss the effect of surface roughness on these reflections and show that only for two very flat surfaces can imaging be successful, but that in general profiling of the diffracted beam allows accurate statistical measurement of surface roughness. We use this latter method to probe interface roughness at conventional Si/Si0 2 interfaces using plan-view diffraction, with greater accuracy than can be claimed for high-resolution microscopy.[3' 4 ] With in-situ UHV studies we watch the interaction of 02 with clean Si surfaces, directly revealing the behavior of individual atomic steps by application of the former technique. These results show that the room temperature oxidation
process occurs on terraces and not preferentially
at steps, as had been
suggested. When a single layer of a crystal structure possesses Fourier components which are absent in a complete unit cell (a very common occurrence), then the potential of forbidden surface-sensitive reflections exists. A simple example is the 1/3 422 for F.c.c. crystals with (111) surfaces. Each (111) layer A(n=l), B(n=2), C(n=3) diffracts with intensity Fe2,rn 3 so that a complete unit cell has zero intensity. However a foil of thickness t*3m layers will give rise to some intensity, as seen for Au by Cherns.[ 2' Furthermore, dark-field images taken with the reflections reveal single atomic steps on both surfaces, since a change in thickness of one monolayer can change the intensity from 0 to f2 . Although Cherns was successful in using this technique to image surface steps on Au, and success has been achieved in one "exceptionally thin and flat specimen" for Si,[61 the application of this promising method to a wide variety of interfaces and surfaces has proven difficult.[71 We will explain this further in terms of surface roughness but first will show that for clean flat Si surfaces prepared by in-situ UHV cleaning, step imaging is readily possible for Si and has been successfully applied to the study of Si oxidation. Si samples cut from 10 K2cm- 1 p-type wafers were thinned HF/HNO3 etching and kept in deionized water prior to insertion in 200CX UHV transmission electron microscope (TEM). The in-situ involves heating to -1200*C and is described elsewhere.J 8] Oxygen
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